Circadian rhythms and environmental lighting regulate a number of endocrine and behavioral functions. Dispersed chick pineal cells remain rhythmic and responsive to light in culture. Light has two apparent effects on the melatonin rhythm displayed by these cells: the first is an acute inhibition of melatonin output, the second is entrainment of the underlying pacemaker. Pertussis toxin, which acts to block the function of transducin and Gi, blocked acute effects of light but not its entraining effects. There must, therefore, be at least two mechanistic pathways for photoendocrine transduction in the chick pineal. The mechanisms of phototransduction appear to differ from those in retinal rod cells. Light and calcium influx appear to regulate melatonin production (acutely) via cyclic AMP. The mechanisms of photoentrainment remain unknown.